Discrimination of trackball-produced forces

The ability of subjects to discriminate forces counter to user movement generated from a trackball with force feedback was examined as an underlying factor in the interpretation of tactually displayed forms. Subjects were required to report, using an adaptive two-alternative forced-choice method (2AFC), which of two sequentially presented stimuli felt weaker. In the first experiment, three reference force levels of 0.88 N, 0.99 N and 1.10 N by three movement displacements of 20°, 40° and 60° were presented. A second experiment was conducted to examine discrimination from reference forces below 0.88 N and to consider the influence of movement orientation. Results from two experiments indicated that ‘just noticeable differences’ (JNDs) in a range from 0.4 to 1.1 N were relatively constant at a level of 15%. In the lower force range, 0.2-0.4 N, JNDs were observed to increase with decreasing force level. The movement amplitude and orientation, whether horizontal or vertical, appeared to have no effect on the observed JND. The experiments showed that the discrimination of forces correlated with the subject's movement speeds within the upper force range.